This invention pertains generally to the field of micromechanical devices and processing techniques, and particularly to micromechanical actuators and actuators for fiber optic switches.
Linear microactuators and micromotors are utilized in a variety of applications in micro-electromechanical systems (MEMS), including incorporation in electrical switches, relays, valves and rotational drives. A significant application of linear microactuators is in switches that are utilized to send a signal on an incoming optical fiber to one of two output fibers (i.e., a 1xc3x972 switch). Such devices may be utilized as network elements in optical communication systems, subscriber loop networks, fiber to the home applications, optical cross-connects for redundant or protective switching, and for factory testing of optical elements and equipment. See, generally, H. Jones-Bey, xe2x80x9cOptical Switches Pursue Cross-Connect Markets,xe2x80x9d Laser Focus World, May, 1998, pp. 153-162.
There are two general approaches to accomplish optical switching with the use of passive optical techniques (that is, without using optical amplifiers). One approach is to move the incoming fiber itself into alignment with one or the other of the two outgoing optical fibers (moving fiber switch). The other approach fixes both the input and output fibers in place and moves something else that routes the light to one of the two outgoing fibers (fixed fiber switch). Of the two approaches, the moving fiber switch has the potential to obtain a lower insertion loss because the fixed fiber switches require additional elements within the switch, such as a mirror or waveguide, which result in additional optical losses. Switch losses originate from factors such as fiber misalignment and the Fresnel reflection loss at the glass-air interface. As a consequence, the moving fiber type of switch is particularly attractive for use in low loss applications. For all of these applications, it is generally preferable to switch a single mode optical fiber because it has less optical power lost per unit length as compared to a multimode fiber. The task of switching a single mode optical fiber is challenging because the core of a single mode fiber is small (e.g., about 9 xcexcm in diameter), and this small size makes precise alignment of the fibers within the switch extremely critical. Low loss switching (less than 1 dB optical power loss) requires alignment tolerance on the order of tenths of microns or less. The ability to properly align the fibers is thus the most critical issue affecting optical switching performance.
Moving fiber switches have been fabricated using several techniques. Because the throw requirement is somewhat greater than the diameter of one fiber (about 125 xcexcm), the switches that have been implemented have typically used thermal and magnetic actuators. Surface micromachining techniques have been used to fabricate an optical fiber switch with latching obtained by using two actuators. M. Hoffmann, et al., xe2x80x9cOptical Fiber Switches Based on Full Wafer Silicon Micromachining,xe2x80x9d J. Micromechanics and Microengineering, Vol. 9, 1999, pp. 151-155. The power dissipated in thermal actuators is relatively large (on the order of several hundred milliwatts). This power dissipation can be troublesome when a large number of switches are employed. In contrast, magnetic switches have been used to produce excellent low power switches. N. Tabat, et al., xe2x80x9cSingle Flux-Path Bi-Directional Linear Actuators,xe2x80x9d HARMST ""97, Madison, Wis., June, 1997; and Tabat, et al., U.S. Pat. No. 5,808,384. Such devices have been produced utilizing the LIGA microfabrication process, which produces parts which have not only high precision tolerances but extremely low run-out as well. As a consequence, parts made using the LIGA technology have virtually perfect vertical sidewalls. Typical run-out for LIGA parts is less than 0.1 xcexcm per 100 xcexcm of height, which is advantageous for fabricating alignment fixtures for the optical fiber switch. LIGA can be used to make both durable metal parts as well as soft magnetic materials. See, e.g., T. R. Christenson, et al., xe2x80x9cApplication of Deep X-Ray Lithography Fabricated Rare Earth Permanent Magnets to Multi-Pole Magnetic Microactuators,xe2x80x9d Transducers ""99, June, 1999. Additional microactuators fabricated utilizing the LIGA process are described in T. Earles, et al., xe2x80x9cMagnetic Microactuators for Relay Applications,xe2x80x9d Proc. of Actuator 96, Jun. 26-28, 1996, Bremen, Germany, pp. 132-135, and U.S. Pat. No. 5,664,177, entitled Micromechanical Magnetically Actuated Devices.
The optical performance of the moving fiber type switches depends almost entirely on the quality of the fiber alignment. This alignment is done typically through the use of some type of V-groove technique. The precision of the V-groove therefore determines the quality of the switch. Earlier versions of LIGA process fabricated optical fiber switches yielded excellent optical performance (0.5 dB insertion loss in air) because the technology produces excellent alignment flats for the fiber. See, H. Guckel, et al., xe2x80x9cSingle Mode Optical Fiber Switch,xe2x80x9d HARMST ""99, Tokyo, Japan, June, 1999. This performance level was achieved even without the use of matching fluids. Generally, avoiding the use of matching fluid is attractive to avoid packaging problems and reliability issues. However, when matching fluid was used, the insertion loss for such devices was reduced to the order of 0.1-0.2 dB. A limitation of prior versions of LIGA optical switches was the requirement for the continuous application of power to hold the moving fiber in either of its two end positions. Where the switch is to be used in applications in which switching will take place relatively infrequently, it would generally be desirable to have a switch which is latched in its two end positions and requires power only during switching to reduce overall power consumption and device heating.
A bi-directional micromechanical latching linear actuator in accordance with the invention provides high precision linear actuation for applications such as electrical and optical switches, relays, valves, and other devices requiring high precision actuation. Using micromachining techniques, the actuator may be embodied in a physical structure having dimensions of a few millimeters on a side or less. The actuator action provides relatively high force while providing relatively low electrical impedance, allowing low drive voltages to be utilized. Electrical power is required only during switching of the actuator from one of its end positions to the other, with the actuator remaining latched in its end position after switching has been completed with no further drive power required. Consequently, the total power consumption for operation of the device is very low, and during periods of time when no switching occurs, no power is consumed.
The bi-directional micromechanical latching actuator of the invention includes a non-magnetic substrate having a surface, a plunger having two magnetic heads spaced from each other and joined to move together, with at least the heads of the plunger formed of a magnetic material, and a magnetic core supported on the substrate having first end faces spaced apart to define a first gap in the core adjacent to a first of the heads of the plunger and second end faces spaced apart to define a second gap in the core adjacent to a second of the heads of the plunger. Means are provided for supporting the plunger for linear movement in two directions such that the heads of the plunger can move toward and away from the first and second gaps in the core. Motion in one direction brings the first of the heads closer to the first of the gaps in the core and the second of the heads further from the second of the gaps in the core. Motion in the other direction brings the second head closer to the second gap in the core and the first head further away from the first gap in the core. At least one permanent magnet is mounted to the plunger to move therewith and forms with the core a first magnetic circuit in which flux from the permanent magnet passes through the magnetic core across the first of the gaps through the first head of the plunger and then back through the magnetic core to the permanent magnet. A second magnetic circuit is formed in which the flux from the permanent magnet passes through the magnetic core across the second of the gaps in the core through the second of the heads and thence back through the magnetic core to the permanent magnet. At least one coil of electrical conductor is coupled to the magnetic core to provide magnet flux therethrough to the first and second magnetic circuits. When the coil is supplied with electrical current in a first direction, the coil provides flux in a direction through the first magnetic circuit which augments the flux from the permanent magnet such that the first head of the plunger is magnetically drawn toward the first gap by reluctance action and provides flux to the second magnetic circuit in a direction to oppose the flux from the permanent magnet. When the direction of current through the coil is reversed, the coil provides flux to the first magnetic circuit to oppose the flux from the permanent magnet and provides flux to the second magnetic circuit which augments the flux from the permanent magnet such that the second head of the plunger is magnetically drawn toward the second gap by reluctance action. The current through the coil is preferably selected to substantially null the flux in one of the gaps, with the increased flux through the other gap providing a strong magnetic force on the adjacent head of the plunger to rapidly switch the plunger.
For high precision switching operations, the actuator may be provided with stop structures positioned to engage a portion of the plunger at a selected limit of travel of the plunger in each direction of linear movement of the plunger. In this manner, the plunger is held at one or the other of its limits of travel by reluctance action from the flux from the permanent magnet when no current is supplied to the first and second coils with the position of the plunger being precisely fixed.
A means for supporting the plunger may comprise a spring mounted to the substrate to suspend the plunger for linear movement above the substrate surface. The spring provides a relatively frictionless support for the plunger and provides a spring bias of the plunger back to a neutral position in which each of the heads of the plunger are withdrawn an equal distance from the adjacent gaps. This spring bias provided by the spring further augments the force applied to the plunger to switch it from one of its positions to the other when the coil is supplied with power.
The actuator of the invention is particularly adapted to utilization with an optical switch coupled to the plunger to shift the direction of transmission of light through the switch when the plunger is moved from one of its limits of travel to the other. Such an optical switch may comprise an optical fiber connected to the plunger to be moved by it and two fixed optical fibers, the moving and fixed optical fibers having end faces such that the end face of the moving fiber is aligned with the end face of one or the other fixed fiber at each of the limits of travel.
Further objects, features and advantages of the invention will be apparent from the following detailed description when taken in conjunction with the accompanying drawings.